limit candidate quiz dropdown to only show "active" quizzes

completes #105

app/controllers/admin/candidate_controller.rb

@@ -61,7 +61,7 @@ module Admin
       end
 
       def collect_quizzes
-        @quizzes ||= Quiz.order(:name)
+        @quizzes ||= Quiz.all_active.order(:name)
       end
 
       def send_notifications candidate

app/models/quiz.rb

@@ -8,4 +8,10 @@ class Quiz < ApplicationRecord
   validates :name, presence: true, uniqueness: true
   validates :dept, presence: true
   validates :unit, presence: true
+
+  class << self
+    def all_active
+      Quiz.joins(:questions).distinct
+    end
+  end
 end